there is a school of thought who go for fortnightly cleaning-  when i first meet customers, i Ask Them how often ,rather than suggesting monthly myself,otherwise im saddled with monthly . 
I usually say-- "how oftenwould u like ?,nearly all  is a fortnight (isnt true) but a few are every 4weeks (mutter the last bit ) "      . by saying it this way it filters out the "draggers " who really wanted to be every couple of months but now think they are being  a bit draggey by becoming monthlys 
my price is the same whether fortnightly or monthly or 6 weekly.